Venous thromboembolism (VTE) is one of the most common, life-threatening cardiovascular diseases in the US, causing over 350,000 hospitalizations each year. The rate of VTE has been rising over time and is expected to increase even further as the US and worldwide population ages. The cornerstone of VTE treatment is anticoagulation, which while very effective, is associated with significant bleeding risks and burdensome monitoring. Adverse complications from anticoagulation are particularly serious in older adults and individuals with cancer, who are also the patients at highest risk for recurrent VTE. Effective risk stratification tools that can optimally balance recurrent VTE risk with hemorrhagic complications of treatment have yet to be developed, and lack of data on the safety of extended-duration anticoagulant therapy and effect on long-term outcomes leads to controversy about the optimal duration of therapy after initial VTE. Establishing strategies that can better inform best practices in the prevention and treatment of VTE is of major public health importance. The long-range objective of this project is to reduce the morbidity and mortality associated with VTE in adults and evaluate contemporary treatment patterns and long-term outcomes after VTE. This project will create a richly-detailed longitudinal cohort of adults with first-time VTE diagnosed in years 2004-2007, based on an administrative registry of VTE patients developed through a collaboration of 4 geographically diverse health plans participating in the National Heart, Lung, and Blood Institute-sponsored Cardiovascular Research Network (CVRN). The goals of this current application are to augment and update this administrative database with important information on clinical conditions, treatment patterns, and long-term outcomes after initial VTE until year 2013, obtained through detailed chart review and outcome validation. The specific aims of this study are to identify factors that influence the type, duration, and quality of initial anticoagulant treatment for VTE, quantify the long-term risk for recurrent VTE and major hemorrhagic outcomes, and develop clinical risk stratification tools that can be used to predict recurrent VTE and major hemorrhage. Combining clinical and outcome data with administrative data will create a cohort of VTE patients with extended follow-up that can serve as a rich source of information for use in facilitating comparative effectiveness research addressing optimal VTE management within contemporary, real-world practice settings.